Video playback on screens that include both the video and a computer interface forming a frame around the video being played, such as with a video playback application for example, are very common. With the advent of very high quality images, one of the video planes (or layers) may be provided in a high quality format such as high dynamic range (HDR) while the user interface layer may be provided in a lower quality format such as standard dynamic range (SDR). HDR video playback is very popular and is fast becoming the model for mainstream visual media. Compared to SDR, HDR is often used in imaging and photography to reproduce a greater dynamic range of luminosity. HDR can provide a much greater range of luminance levels covering many real-world scenes containing very bright, direct sunlight to extreme dark or very faint nebulae. Thus, while SDR provides 256 luminance levels from darkest to brightest for each red, green, and blue (RGB) channel and uses eight bits per channel, HDR may provide at least 1024 luminance levels in ten bits for each channel. When an SDR interface layer and HDR video or other content layer of an image are parts of the same image, the two parts are overlaid or blended together to provide image pixel data of a single composite layer or image.
Difficulty arises when the image data for each pixel also includes an alpha (A) value which refers to a value that indicates the transparency level of the pixel. This may be used, for example, when the interface has features that overlay the video but are at least somewhat transparent so that the video underneath can be seen through the features of the interface. This may include a play button or a pause button placed over the middle of the video for example. This RGBA color space or model is often provided in SDR with 8 bits per channel to provide a 32 bit word that can be aligned in memory whether cache, RAM, or other memory. To blend the two layers, the SDR layer is first converted to HDR. This, however, causes a difficulty since the RGB channels now need 10 bits each and there is insufficient space for the full 8-bit alpha value in the 32 bit word. To resolve this issue, either the alpha value is reduced to two bits so that a 10/10/10/2 bit scheme is provided to fill the 32 bit value which reduces the alpha fidelity and in turn the quality of the image, or a RGB16 format is provided where each of the RGBA channels is provided with a 16 bit value, increasing the bit size to 64 bits per pixel, which consumes a very large memory bandwidth to perform the blending of the two formats. Neither option is acceptable.